1. Technical Field of the Invention
The present invention relates to transmission of telecommunications data in a cellular communication system using spread spectrum modulation and, more particularly, to a method and apparatus for measuring pilot signal strengths and finding multipath delays for Code Division Multiple Access channels in a cellular communication system.
2. Description of Related Art
Spread spectrum communication technology has been used in military communications since the days of World War II, primarily for two purposes; to overcome the effects of strong intentional interference on a certain frequency and to protect the signal from unauthorized access. Both these goals can be achieved by "spreading" the signal spectrum to make it virtually indistinguishable from background noise, hence the term spread spectrum modulation.
Code Division Multiple Access, or CDMA, is a digital cellular spread spectrum multiple access method. In known CDMA systems, a number of base stations are typically located within a service area. Each base station uses one or more CDMA channels to communicate with one or more mobile stations located within the same service area. The base-to-mobile station transmission direction is known as the forward link or downlink and the mobile-to-base station direction is known as the reverse link or uplink.
In a CDMA system, an information data stream to be transmitted is modulated by a data sequence with a much higher data rate, referred to as a "signature sequence". Each element of the signature sequence typically represents one binary logical symbol ("0" or "1"). The signature sequence usually comprises N bits, wherein each of the N bits is denoted as a "chip". One way to generate such a signature sequence is by using a periodic binary sequence of pseudorandom signals to modulate a periodic impulse stream of period T.sub.c, also referred to as "chip duration". The sequence of pseudorandom signals is also known as a pseudo noise (PN) sequence, so called because it appears random but can be replicated by an authorized receiver.
The information data stream and the high bit rate signature sequence are combined by first mapping the binary logical signals ("0" or "1") to real values ("+1" or "-1") and multiplying the two bit streams together. The combination of the lower bit rate information data stream with the higher bit rate signature sequence creates a noiselike wideband signal. This technique is called "coding" or "spreading" the information data stream and is well known in the art.
In traditional cellular communication systems, co-channel interference between channels due to spectrum reuse is one of the main limiting factors in achieving a high system capacity. One of the most notable features of CDMA technology is universal frequency reuse, which means that all users within a CDMA system occupy a common frequency spectrum allocation. This is accomplished by allocating different codes to different channels. On the downlink, each base station transmits a unique, unmodulated spreading code, denoted pilot code, pilot channel or "pilot". The pilot generally consists of a sequence of chips, each having a chip duration T.sub.c. Each pilot is a different shift of a common complex sequence. Hence, on the forward link, each base station transmits a unique, unmodulated pilot channel, and may additionally transmit a synchronization channel, paging channels and traffic channels. The term "CDMA channel set" is used to refer to a set of channels transmitted by a base station.
Each mobile station in a CDMA system searches for pilot codes to detect the presence of base station signals and to measure their strengths. For purposes of this disclosure, a forward CDMA channel set containing one or more traffic channels assigned to the mobile station is referred to as an "active channel", and the pilot signal of such an active channel is referred to as an "active pilot". Conversely, a CDMA channel set which contains no traffic channels assigned to the mobile station is referred to as a "non-active channel", and the pilot signal of such a non-active channel is referred to as a "non-active pilot". Since no traffic information is transmitted from the base station to the mobile station on the non-active channels, there is no need for demodulating these channels. Thus, the mobile station must only be able to demodulate the active CDMA channel sets.
A well-known source of degradation common to all known wireless multiple access systems, particularly in terrestrial environments, is known as "multipath fading". In a multipath environment, the transmitted signal follows several propagation paths from a transmitter to a receiver, typically as a result of the signal reflecting off one or more objects before arriving at the receiver. Since the various propagation paths of the transmitted signal are of unequal lengths, several copies of the transmitted signal will arrive at the receiver with varying time delays. In a multipath fading channel, phase interference between different propagation paths of the transmitted signal may cause severe fading and result in signal dropout or cancellation.
A mobile station in a CDMA system is typically equipped with a receiver for demodulating active channels and compensating for multipath delays as described above. The receiver is generally denoted a RAKE receiver since it "rakes" all the multipath contributions together. A RAKE receiver consists of a number of processing units or RAKE fingers. When demodulating a multipath fading channel, each finger of the RAKE receiver must be synchronized with one of the diverse propagation paths of the channel. A RAKE receiver comprising L fingers is able to detect, at most, L copies of the transmitted signal, which are corrected for time delays and added coherently, (co-phased and scaled). The resulting signal will thus comprise a collection of all the time delayed copies of the transmitted signal.
As previously described, due to multipath propagation the transmitted signals will arrive at different times at the mobile station and hence result in a number of time delayed copies of the transmitted signal at the receiver. The relative time delays of the received copies of the transmitted signal must be determined in order to synchronize the various propagation paths of the signals with the corresponding fingers of the RAKE receiver. Unfortunately, the number and magnitude of the time delays may change due to movement of the mobile station, i.e., variable distance and velocity relative to the transmitting base station for users in motion. Also, movement of the mobile station may cause new channel paths to appear and old channel paths to disappear. Hence, the mobile station must continuously monitor the signals received along all propagation paths of an active channel in order to search for new, stronger channel paths. To perform this monitoring efficiently, the multipath time delays must be substantially continually measured or estimated in a fast and accurate manner.
In a cellular system conforming to the TIA/EIA/IS-95 "Mobile station--base station compatibility standard for dual mode wideband spread spectrum cellular system" standard, the ANSI J-STD-008 "Personal station--base station compatibility requirements for 1.8 to 2.0 GHz code division multiple access (CDMA) personal communications systems" standard, or other similar standards, a mobile station must be able to switch connection from a first base station to a second base station if the second base station provides a stronger signal to the mobile station, a procedure known as "handoff". As described in the standards documentation, the handoff may be accomplished through either a "hard" handoff or a "soft" handoff.
In hard, or traditional, handoff, the connections to different base stations use different frequencies, which means that the connection to the old base station is broken before the connection to the new base station is set up. However, because of the universal frequency reuse in a CDMA system, it is possible to set up the connection to a new base station before leaving the old base station, a procedure known as soft handoff. According to the above-identified standards, the mobile station must continuously measure the signal strengths for all received pilots in order to decide if a handoff, either hard or soft, is required.
Both of the two functions just described, i.e., searching for stronger paths for active channels using time delay estimates and continuously measuring pilot signal strengths for received channels, are typically performed by a circuit in the mobile station generally denoted a "searcher". The searcher specifies a window of correlation, also referred to as a search window, for each received pilot signal. The search window consists of a predetermined number of consecutive chips among which the probability of finding usable multipath components of the corresponding channel is high. To specify the search window for a particular pilot, a locally generated replica (local pilot) of that particular pilot is used for correlation with the received pilot. The search window is centered around the earliest arriving usable multipath component (correlation peak) of the pilot, which occurs when the received pilot matches the locally generated pilot. The search window further employs a search range of W chips on either side of the center, where W is a predefined number as specified in either of the standards TIA/EIA/IS-95 or ANSI J-STD-008 as referred to above.
The conventional technique for performing the correlation described above is by using an "integrate and dump" correlator, which integrates the received signals during a given time period and then resets itself. The correlation peaks for each search window are detected, and the results are used to calculate the pilot strength for the corresponding pilots. Also, the estimated multipath delays for the active channels to be demodulated are calculated based on the correlation results. These estimated multipath delays are used to synchronize each finger of the RAKE receiver with one of the propagation paths of the active channels as previously described.
Prior searcher techniques typically employ integrate and dump correlation for each separate multipath delay in each search window. Such a method is calculation intensive which makes the search process relatively slow. Consequently, valuable time which could be used to improve the accuracy of the search results is wasted.
Accordingly, there is a need in the art for an improved method and apparatus for searching for stronger paths for active CDMA channels while continuously measuring pilot signal strengths for all received CDMA channels. The present invention uses new techniques to generate and process a search window for each pilot in the system, resulting in faster and more accurate measurements.